I have and idea and thought I would ask the "all knowing" frame builders what they though.

I have a small but noticeable dent in my toptube from a garage door coming down on it. My thought is to heat that area of the frame with a heatgun and then cool it quickly with dry ice. I know this works with thinner metal like car door panels and what not. What about thicker metal like an aluminum frame?

Aluminum conducts heat too well for the torch/ice to only affect one small area. I doubt this would work on steel tube either. A tube just seems too rigid of a structure. 18g sheet is fairly flimsy which is why the ice trick works on large panels. There can't be any harm in trying, though. How hot does your heatgun get? I don't think you'll harm the Al if you keep it under 300F.

The old steel frame dent removal trick might work on your Al frame as well...

Get a block of hardwood, like oak, and drill a hole in it the same size as your frame tube. Cut the wood in half so there are channels in each piece. Clamp the blocks with the channels over the dented area. (assuming there are no cable guides in the way) As you slowly tighten the vice, twist the frame so that the dented tube is moving back & forth in the hole. Keep tightening and twisting. Eventually, most of the dent will be pushed back out. The rest you can fill with a little Bondo.